View Full Version : Wood floor underlayment

Craig Summers
08-02-2008, 1:02 AM
Looking for some advice. I am remodelling my 12x24 family room, and am preparing to install Bruce engineered (1/2" plywood) oak floor.

The existing subloor is joists, 1/2" plywood and 3/4 MDF on top. Is it a good idea to nail the oak floor to the MDF & Plywood or should it be glued? Or should the MDF be replaced?

I plan to install the Oak floor parallel to the joists, the 12' direction.

Joe Chritz
08-02-2008, 9:11 AM
Probably should check with the manufacturer on that. I have had to replace particle board to go under plank flooring because of lack of holding power of the nails. The engineered stuff may be fine with an MDF underlayment.

That 1/2 ply, 3/4" engineered stuff seems to be common on some of the older homes in my area. Must have been a sale at some point.


Tom Godley
08-02-2008, 10:07 AM
I have also seen the use of 1/2 ply and then the mdf on top ??
What is the reason for this? -- why not just 3/4 ply?? It can not be cheeper!

The use of glue for flooring seems to be widespread today -- I have never used it as the primary holder -- only because I never want to deal with it if I need to take it up. But I think that glue may be best over the mdf. The holding power may be a problem with it down.

I normally like the floor to flow with the long wall -- Even when this requires a transition between rooms.

Craig Summers
08-03-2008, 1:18 AM
On the direction of the floor, SWMBO said put floor in the short way. She and MIL says that the long way makes the room look longer, "like a bowling alley" and there is a half wall into the kitchen. The crown jewel for the family room project is the 42" flat screen over the fireplace.

I might have to just test the nail thing. Guess i'll try my finish nail gun before i rent a nailer.

Oh and yes, the house has changed hands a few times. It is 36 years old(as old as me), and we got it in 2005 just after the bubble popped. It appears thats the MDF was an after thought, as the stud walls were built only on the 1/2" underlayment. Most of the house (except family room, kitchen and bathrooms) had solid oak floors that were hidden under carpet, but are now refinished.

Pat Germain
08-03-2008, 10:01 AM
I did a lot of reading before I installed installed my hardwood flooring recently. Based on my research, plywood and OSB are fine for most hardwood flooring when using nails or staples. However, MDF is not. As mentioned, engineered flooring may be different.

Mike Cutler
08-03-2008, 8:19 PM

You seem to be a step behind me in the home remuddling phase.

I also will be installing the finished floor parallel to the floor joists. All the internet research I did recommended that the flooring be ran across the joists, unless the subfloor was beefed up, and made flat in all directions.
I used a 3/4" layer of Georgia Pacific engineered subfloor, with a 1/2"" ply layer on top of that. The subfloor was screwed and glued to the joists and the 1/2" ply layer was set on top and screwed down on 6" centers.

Ben Grunow
08-03-2008, 9:35 PM
The big risk of running flooring parallel to the joists is the wave effect if the subfloor sags. The flooring spans the joists if installed correctly and eleminates this problem. Mike will probably not have this problem with his built up subfloor but that is a lot of work just to have the floor boards go the way you want them to.

Matt Ocel
08-03-2008, 10:14 PM
Craig -
FYI the 1/2 plywood would be your sub floor, the mdf is considered underlayment, when I first started building houses (bout 1987) thats the way we did it, now its 3/4" t and g. I believe that back in the day 3/4" t&g wasn't readily available.

Anyway assuming the mdf is in good shape and you use staples, then have at it, you'll be fine and with 1 1/2" under your finished floor I would be comfortable running it in any direction.

Send pix when completed.

P.S. Mike, a little over kill. But in this case more is better.

Jim Becker
08-03-2008, 10:41 PM
MDF isn't going to give you much to nail to. Perhaps you should consider floating the floor or doing a glue-down installation rather than nailing. Ripping up the MDF will be "real work".